update code to reflect the new minimum python version 3.7

- update string format method
- remove redundant default arguments from function  calls
- remove unused imports
- remove unicode identifier from strings, it's default in py3 (see: https://stackoverflow.com/a/4182635/12201331)

allianceauth/fleetactivitytracking/migrations/0001_initial.py

@@ -1,5 +1,4 @@
 # Generated by Django 1.10.1 on 2016-09-05 21:39
-from __future__ import unicode_literals
 
 import django.db.models.deletion
 from django.conf import settings
@@ -54,6 +53,6 @@ class Migration(migrations.Migration):
         ),
         migrations.AlterUniqueTogether(
             name='fat',
-            unique_together=set([('character', 'fatlink')]),
+            unique_together={('character', 'fatlink')},
         ),
     ]

allianceauth/fleetactivitytracking/migrations/0002_auto_20160905_2220.py

@@ -1,5 +1,4 @@
 # Generated by Django 1.10.1 on 2016-09-05 22:20
-from __future__ import unicode_literals
 
 import datetime
 from django.db import migrations, models

allianceauth/fleetactivitytracking/migrations/0003_auto_20160906_2354.py

@@ -1,5 +1,4 @@
 # Generated by Django 1.10.1 on 2016-09-06 23:54
-from __future__ import unicode_literals
 
 from django.db import migrations, models
 import django.utils.timezone

allianceauth/fleetactivitytracking/migrations/0004_make_strings_more_stringy.py

@@ -1,5 +1,4 @@
 # Generated by Django 1.10.5 on 2017-03-22 23:35
-from __future__ import unicode_literals
 
 from django.db import migrations, models